1. Field of the Invention
The present invention relates generally to multi-piece, golf balls having a solid core comprising layers made of foam and thermoset compositions. Particularly, the multi-layered core has a foam inner core (center) and surrounding thermoset core layers. The core layers have different hardness gradients and specific gravity values. The core assembly preferably has a positive hardness gradient extending across the entire assembly. The ball further includes a cover of at least one layer.
2. Brief Review of the Related Art
Professional and recreational golfers normally play with multi-piece, solid golf balls. Such balls typically include an inner core made of a natural or synthetic rubber such as polybutadiene, styrene butadiene, or polyisoprene. The ball further includes a cover surrounding the inner core. The cover protects the inner core and makes the ball more durable. The outer cover may be made from a variety of materials including ethylene acid copolymer ionomers, polyamides, polyesters, polyurethanes, and polyureas. The ball may further include one or more intermediate layers disposed between the inner core and outer cover.
Manufacturers of golf balls use different materials to impart specific features to the ball. For example, the resiliency and rebounding performance of the golf ball are important properties and are based primarily on the composition and construction of the core. The core acts as an engine or spring for the ball. In general, the rebounding performance of the ball is determined by calculating its initial velocity after being struck by the face of the golf club and its outgoing velocity after making impact with a hard surface. More particularly, the “Coefficient of Restitution” or “COR” of a golf ball refers to the ratio of a ball's rebound velocity to its initial incoming velocity when the ball is fired out of an air cannon into a rigid vertical plate. The COR for a golf ball is written as a decimal value between zero and one. A golf ball may have different COR values at different initial velocities. The United States Golf Association (USGA) sets limits on the initial velocity of the ball so one objective of golf ball manufacturers is to maximize COR under such conditions. Balls with a higher rebound velocity have a higher COR value. Such golf balls rebound faster, retain more total energy when struck with a club, and have longer flight distance.
The durability, spin rate, and feel of the ball also are important properties. In general, the durability of the ball refers to the impact-resistance of the ball. Balls having low durability appear worn and damaged even when such balls are used only for brief time periods. In some instances, the cover may be cracked or torn. The spin rate refers to the ball's rate of rotation after it is hit by a club. Balls having a relatively high spin rate are advantageous for short distance shots made with irons and wedges. Professional and highly skilled amateur golfers can place a back spin more easily on such balls. This helps a player better control the ball and improves shot accuracy and placement. By placing the right amount of spin on the ball, the player can get the ball to stop precisely on the green or place a fade on the ball during approach shots. On the other hand, recreational players who cannot intentionally control the spin of the ball when hitting it with a club are less likely to use high spin balls. For such players, the ball can spin sideways more easily and drift far-off the course, especially if it is hooked or sliced. Meanwhile, the “feel” of the ball generally refers to the sensation that a player experiences when striking the ball with the club and it is a difficult property to quantify. Most players prefer balls having a soft feel, because the player experience a more natural and comfortable sensation when their club face makes contact with these balls. Balls having a softer feel are particularly desirable when making short shots around the green, because the player senses more with such balls. The feel of the ball primarily depends upon the hardness and compression of the ball.
Manufacturers of golf balls are constantly looking to new materials for improving the playing performance properties of the ball. For example, Puckett and Cadorniga, U.S. Pat. Nos. 4,836,552 and 4,839,116 disclose one-piece, short distance golf balls made of a foam composition comprising a thermoplastic polymer (ethylene acid copolymer ionomer such as Surlyn®) and filler material (microscopic glass bubbles). The density of the composition increases from the center to the surface of the ball. Thus, the ball has relatively dense outer skin and a cellular inner core. According to the '552 and '116 patents, by providing a short distance golf ball, which will play approximately 50% of the distance of a conventional golf ball, the land requirements for a golf course can be reduced 67% to 50%.
Gentiluomo, U.S. Pat. No. 5,104,126 discloses a three-piece golf ball (FIG. 2) containing a high density center (3) made of steel, surrounded by an outer core (4) of low density resilient syntactic foam composition, and encapsulated by an ethylene acid copolymer ionomer (Surlyn®) cover (5). The '126 patent defines the syntactic foam as being a low density composition consisting of granulated cork or hollow spheres of either phenolic, epoxy, ceramic or glass, dispersed within a resilient elastomer.
Aoyama, U.S. Pat. Nos. 5,688,192 and 5,823,889 disclose a golf ball containing a core, wherein the core comprising an inner and outer portion, and a cover made of a material such as balata rubber or ethylene acid copolymer ionomer. The core is made by foaming, injecting a compressible material, gasses, blowing agents, or gas-containing microspheres into polybutadiene or other core material. According to the '889 patent, polyurethane compositions may be used. The compressible material, for example, gas-containing compressible cells may be dispersed in a limited part of the core so that the portion containing the compressible material has a specific gravity of greater than 1.00. Alternatively, the compressible material may be dispersed throughout the entire core. In one embodiment, the core comprises an inner and outer portion. In another embodiment, the core comprises inner and outer layers.
Sullivan and Binette, U.S. Pat. No. 5,833,553 discloses a golf ball having core with a coefficient of restitution of at least 0.650 and a cover with a thickness of at least 3.6 mm (0.142 inches) and a Shore D hardness of at least 60. According to the '553 patent, the combination of a soft core with a thick, hard cover results in a ball having better distance. The '553 patent discloses that the core may be formed from a uniform composition or may be a dual or multi-layer core, and it may be foamed or unfoamed. Polybutadiene rubber, natural rubber, metallocene catalyzed polyolefins, and polyurethanes are described as being suitable materials for making the core.
Sullivan and Ladd, U.S. Pat. No. 6,688,991 discloses a golf ball containing a low specific gravity core and an optional intermediate layer. This sub-assembly is encased within a high specific gravity cover with Shore D hardness in the range of about 40 to about 80. The core is preferably made from a highly neutralized thermoplastic polymer such as ethylene acid copolymer which has been foamed. The cover preferably has high specific gravity fillers dispersed therein.
Nesbitt, U.S. Pat. No. 6,767,294 discloses a golf ball comprising: i) a pressurized foamed inner center formed from a thermoset material, a thermoplastic material, or combinations thereof, a blowing agent and a cross-linking agent and, ii) an outer core layer formed from a second thermoset material, a thermoplastic material, or combinations thereof. Additionally, a barrier resin or film can be applied over the outer core layer to reduce the diffusion of the internal gas and pressure from the nucleus (center and outer core layer). Preferred polymers for the barrier layer have low permeability such as Saran® film (poly(vinylidene chloride), Barex® resin (acyrlonitrile-co-methyl acrylate), poly(vinyl alcohol), and PET film (polyethylene terephthalate). The '294 patent does not disclose core layers having different hardness gradients.
Sullivan, Ladd, and Hebert, U.S. Pat. No. 7,708,654 discloses a golf ball having a foamed intermediate layer. Referring to FIG. 1 in the '654 patent, the golf ball includes a core (12), an intermediate layer (14) made of a highly neutralized polymer having a reduced specific gravity (less than 0.95), and a cover (16). According to the '654 patent, the intermediate layer can be an outer core, a mantle layer, or an inner cover. The reduction in specific gravity of the intermediate layer is caused by foaming the composition of the layer and this reduction can be as high as 30%. The '654 patent discloses that other foamed compositions such as foamed polyurethanes and polyureas may be used to form the intermediate layer.
Tutmark, U.S. Pat. No. 8,272,971 is directed to golf balls containing an element that reduces the distance of the ball's flight path. In one embodiment, the ball includes a core and cover. A cavity is formed between core and cover and this may be filled by a foamed polyurethane “middle layer” in order to dampen the ball's flight properties. The foam of the middle layer is relatively light in weight; and the core is relatively heavy and dense. According to the '971 patent, when a golfer strikes the ball with a club, the foam in the middle layer actuates and compresses, thereby absorbing much of the impact from the impact of the ball.
One disadvantage with golf balls having a foam core is the ball tends to have low resiliency. That is, the velocity of the ball tends to be low after being hit by a club and the ball generally travels short distances. Golf balls having foam inner cores are often referred to as reduced distance balls. There is a need for new balls having a foam core with improved resiliency that will allow players to generate higher initial ball speed. This will allow players to make longer distance shots. The present invention provides new foam core constructions having improved resiliency as well as other advantageous properties, features, and benefits. The invention also encompasses golf balls containing the improved core constructions.